TreeSet mapReduce implementation
Merge branch 'master' into topic/wip-trees-ops

Conflicts:
	src/main/scala/scala/collection/parallel/workstealing/Trees.scala
HashTrieSet reducable and aggregate based ops
fixing errors in map-reduce implementation(combining node with itself)
TrieSet reduce& aggregate based operations tests
TrieMap.mapReduce implemented.
TrieMap.mapReduce and aggregate based operations
HashTrieSet.flatMap and filter implemented
WIP on hashSet tests
ParHashSet tests finished
WIP on HastTrieMap filter and flatMap
fixing bug in hashTrieMap.aggregate
Fixing errors in HashSet tests
HashTrieSet and HashTrieMap tests finished.
Thought they fail with methods inherited from Reducable, it seems that there's a bug either in splitter or in reducable methods
Custom find methods for HashTrieMap and HashTrieSet
Test to reproduce HashTrieSet as reducable failure
Wip on ParHashSet ops
HashSet find, exists and forall
HashSet ops&tests finished
HashSet as Reducable tests
Merge branch 'topic/wip-trees-ops' into topic/wip-reducables-comparison

Conflicts:
	src/test/scala/scala/collection/parallel/ParHashSetSnippets.scala
	src/test/scala/scala/collection/parallel/scalatest/ParHashSetTest.scala